It is known that batch furnaces or crucible furnaces or continuous canal furnaces, which differ from each other as regards the procedures for the melting process, may be used for the production of vitreous materials, such as, for example, a mosaic product composed of a vitreous paste.
In batch furnaces, the raw materials contained in the crucible are firstly heated to a high temperature in order to melt them and form the vitreous mixture, and other raw materials such as, for example, silica sand, are then added in order to obtain an opacifying effect and a crystalline grain; finally, said materials are cooled, before being conveyed to suitable forming machines in order to produce the end product, for example a vitreous mosaic material.
Owing to these process characteristics, crucible furnaces are suitable for small production outputs, ranging between 100 and 3000 kg of vitreous product per day. In continuous furnaces, the various stages of production are distributed spatially, but are performed simultaneously. The raw materials forming the primary vitrifiable mixture are melted continuously inside a tank which is connected by means of submerged passage or gully to a canal. Substances necessary for producing the opacifying effect are added into the canal. A casting tank which supplies the forming machines is situated at the other end of the canal.
Unlike crucible furnaces, continuous furnaces are suitable for greater production outputs, exceeding 5000 kg of vitreous product per day.
Ceramic frits are produced industrially in furnaces of the continuous type. At present melting furnaces of the oxygen-combustion type are in particular preferred. In view of their small dimensions, for these applications the use of efficient, but costly systems for pre-heating the comburent air, such as regenerators, as used in large glass tank furnaces, is avoided. Thus, the fumes are conveyed directly to the flue, still at a high temperature. Owing to the notable environmental impact of the flue emissions, in accordance with recent legislation, furnaces for ceramic frits must also be equipped with a fume filtration device of the sleeve filter type. These plants not only have a high installation cost, but are also costly to manage on account of the large volume of fumes due also to mixing with the ambient air necessary in order to lower the temperature to levels compatible with the filters used.
Furnaces for vitrification of waste at present constitute a type of plant which is still in the experimental stage. The raw materials which form the vitrifiable mixture consist, wholly or partly, of toxic waste of inorganic origin, such as for example the residual matter from RSU incinerators and the dross resulting from the processing of metals and composite materials containing asbestos. The aim of this type of treatment of dangerous waste is to produce glass which has a suitable chemical stability and which, even though not completely refined, may be reused as a semi-processed product in the ceramics, glass fibers and foamed glass industry for thermal insulation or files to be used in the building sector.
In all the abovementioned production processes, the method of melting the vitreous products, such as that for ceramics frits and for vitrification of waste, is characterized by the production of glass which is not entirely devoid of internal air bells, namely is not refined. The vitrifiable mixture, moreover, may contain elements which evaporate easily and may therefore have a significant and problematic impact on the environment.
Finally, since the composition of the mixture is subject to frequent changes in order to produce products with a different color and opacifying effect, in order to speed up the material replacement operations, it is preferred to use very low heads of glass.
Generally a drawback of certain solutions consists in the fact that the thickness of the layer of vitrifiable mixture, which is deposited on the surface of the molten bath, is limited and is not sufficient to screen the dispersions which are irradiated towards the crown of the furnace. Thus, some components in the mixture may easily evaporate and mingle with the discharge fumes, thereby contaminating them.
Owing to their high temperature and harmful content, the existing legislation governing pollution requires the use of costly filtration plants.
German patent No. 1,080,740 discloses a furnace for vitreous materials having a tank with a polygonal shape in plan view, suitably designed to ensure a uniform temperature inside the molten bath. Electrodes are mounted on the side walls of the furnace and towards the central zone of the tank and, being suitably energized by electric transformers generate a diffused current within the molten bath. This diffused current heats the vitreous mixture contained in the tank as a result of the Joule effect. During continuous operation, the vitrifiable mixture is deposited on the upper surface of the molten bath so as to form a uniform layer, while an opening on the floor and close to the corner of the tank allows the molten glass to flow out.
A disadvantage of the solution considered consists in the considerable thickness of the head of glass, due to the shape of the tank and the arrangement of the electrodes. This constitutes a limitation when the vitrification mixture must be changed frequently, since it increases the time required for changing the mixture of raw materials to be vitrified.
A second disadvantage of the solution in question consists in the fact that the ends of the electrodes are freely immersed in the molten bath, resulting in a high intensity of current in the vicinity of the said ends. For this reason, the immersed ends of the electrodes are subject to rapid wear.
DE-C-564491 discloses an electric furnace with a plurality of electrodes placed on the floor. Each electrode has a variable cross-section and an interruption in correspondence of a central area of the bath. This interruption defines an internal space in which the convection currents of the melting bath originate. The variable cross-section of the electrodes is specifically directed to provide a vertical extension of the melting bath and does not prevent an increase in the overall head, change time and power consumption of the melting process.
U.S. Pat. No. 4,143,232, which is considered the nearest prior art on which is based the preamble of claims 1 and 5, comprises a glass furnace comprising three groups of electrodes which are positioned in the melting tank at three different levels in order to yield convection currents in the molten bath. The groups of electrodes which are placed at the upper level far from the floor play a very important role during the process as they allow to keep the molten bath flatter and more stable. Moreover, the other two groups of electrodes are placed at different levels and distances from the side walls of the tank in order to improve the control of the convection currents and of the shape of the melting bath. This known furnace has no provision for reducing to a minimum the head of the molten bath, the time for changing the primary batch and the power consumption.